The present invention relates to a device for the extrusion of plastic profiles with an extruder for the melting and delivery of a plastic material and with an extrusion die for defining the profile geometry, whereas a profile slot corresponding to the profile to be extruded and being defined by an outer periphery and by at least one arbor is provided in the extrusion die.
Profiles used in the fabrication of window frames made of plastic are made by extrusion, whereas plastic granules are melted in an extruder and led to an extrusion die provided with a profile slot which corresponds to the geometry of the profile to be extruded. After having passed the extrusion die, such a profile is first sized in dry sizing installations and then submitted to a first cooling phase before it is ultimately cooled down in a water bath or in a spray tank.
An essential part of the measures implied in the production of an extrusion line for a new profile is the tuning. Nowadays, even with the most sophisticated calculating and simulation methods, it is not possible to delineate all the relevant actions taking place in an extrusion die or in an extrusion tool in such a precise manner so that an extrusion tool may be designed in a purely constructive way. In practice it is necessary to first produce the extrusion die and the tools in a first version and then to submit them to a tuning procedure. The tuning, which is made with the original material of the subsequent profile production, analyzes the deficiencies of the manufactured profile and improves it by slight changes operated on the extrusion die and on the extrusion tools. Measures taken during the tuning phase are for example minor alterations in the geometry of the component parts or changes in the temperature distribution within the component parts by additional heating, cooling or insulating. Since, to make such alterations, it is necessary to dismantle the corresponding tools and to finish them accordingly, tuning implies a very complicated procedure.
Thanks to the technological development, the progress that has been made these last years in the field of the productivity of tools is big. It was thus possible to considerably lower the costs for the production of tools. Since however the tuning procedure has essentially remained the same, it still bears considerable saving potential. It is therefore necessary to reduce the number of tuning courses in particular. This desire is particularly urgent with tools intended to be utilized for small or smallest piece numbers. Prototype tools for example are designed for only a few hours of production so that experiments may be made with new profile geometries or with new materials. Such prototype tools may be manufactured in a simple way and at low cost, since they do not require high production speed or long durability. Since the experimental runs are usually executed by skilled personnel, it is not necessary either to optimize them for use or to make them tolerant to operating errors. These very tools however would in particular need extrusion dies which do not or hardly have to be tuned.
A profile tool for an extruder is known from EP 0 593 892. The prior art profile tool has temper elements arranged in its flow channel by means of which the melted plastic material may be heated in a controlled way in some areas of the flow""s cross section. This makes it possible to compensate irregularities due to geometry when the material is passing through the profile slot. With these measures however, it proved very difficult to intervene to good purpose in the production of the profiles. On one hand, the flow conditions between the heating elements and the profile slot itself cannot or only insufficiently be predicted so that it is difficult to decide in which area purposeful heating should take place. On the other hand, the reaction of such a device is very slow, so that the effects of an alteration made can only be fully observed after a considerable lapse of time. Even for persons skilled in extrusion it is extremely difficult to proceed to tuning according to this teaching.
It is an object of the present invention to develop the device described above in such a manner so that the measures implied in tuning can be kept as simple as possible.
According to the invention, a means is provided for the operational alteration of the thickness of the profile slot in at least one predetermined area between the arbor and the outer periphery and another means is provided for cooling selected areas of the extruded profile at the exit of the die. If, for example, in a tuning course, deficiencies in the profile are realized to possibly originate in a too high flow velocity of the material in a determined section of the profile, this very section of the profile slot in the extrusion tool may purposefully be reduced in size. Since this intervention may be made during the extrusion procedure, the measures implied are minimal. If, on the other hand, it has been noticed that the flow velocity of the material in a profile section is too high and that it cannot be influenced by altering the profile slot, the friction resistance in the subsequent sizing tool may be increased by purposeful cooling. Thus, the corresponding profile section is slowed down. These measures will be encountered in particular in complicated profile sections like lining grooves or ribs or profile overhangs.
A particularly simple and advantageous solution is characterized in that the means for altering the thickness of the profile slot is designed in such a manner so as to deform elastically the outer periphery. In this way, ridges or striae are avoided in the extruded profile on the contact surfaces of possibly mobile members.
A particular advantage consists in having at least one part of the outer periphery constituted by a thin-walled web that is mobile for the alteration of the profile slot""s thickness. A particular fine tuning may thus be achieved. The means for altering the thickness of the profile slot is preferably designed as a pressure screw acting upon the outer periphery. With regard to construction, this solution is particularly simple and adequate for prototype tools.
It proved to be sufficient to provide the even profile areas having a bigger surface with a variable profile geometry, in order to come, in most cases, to satisfactory solutions. That is why the means for altering the profile thickness is arranged in an even profile area having a big surface.
In a particularly preferred embodiment of the present invention, the outer periphery is designed in such a manner so that it may be cooled down section after section. As already described above, to achieve profile tuning by thermal influence is already state of the art. The disadvantages of the known solutions, like the slowness of reaction and the difficulty to foresee effects may however be considerably reduced thanks to different measures. In this way, not only the flow resistance of individual profile parts is influenced in the subsequent dry sizing but a certain effect is already produced in the extrusion die. A particular advantage consists in having the means for cooling selected areas of the extruded profile provided, in the area where the profile exits the extrusion die, with nozzles oriented toward the profile and through which air may flow. Cooling by air flow has the advantage that no specific coolant is utilized which would have to be disposed of or recycled after use. It is particularly advantageous to have the nozzles provided together with a device for cooling air to a temperature below the ambient temperature. This method proved to require considerably less air so that a possibly undesired thermal influence on the extrusion die may be reduced.
A particularly compact and stable assembly is achieved by integrating the nozzles in the extrusion die.
On the other hand, improved flexibility is achieved by fastening the nozzles on a front side of the extrusion die. In this solution, the parts surrounding the extrusion die are also submitted to less cooling.
If necessary in order to achieve increased gloss of predetermined parts of the profile""s surface, means for heating selected areas of the profile may additionally be provided at the die""s exit. The easiest procedure is to have the corresponding profile sections blown with hot air.